Conventional optical wavelength dispersive devices, such as those disclosed in U.S. Pat. No. 6,097,859 issued Aug. 1, 2000 to Solgaard et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; U.S. Pat. No. 6,707,959 issued Mar. 16, 2004 to Ducellier et al; U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch; U.S. Pat. No. 6,922,239 issued to Solgaard et al; and U.S. Pat. No. 7,014,326 issued Mar. 21, 2006 to Danagher et al, separate a multiplexed optical beam into constituent wavelengths, and then direct individual wavelengths or groups of wavelengths, which may or may not have been modified, back through the device to a desired output port. Typically the back end of the device includes individually controllable devices, such as a micro-electro-mechanical (MEMs) micro-mirror array, which are used to redirect selected wavelengths back to one of several output ports, or an array of liquid crystal cells, which are used to block or attenuate selected wavelengths.
In the case of a wavelength blocker (WB), or a dynamic gain equalizer (DGE) the front end unit can include a single input/output port with a circulator or one input port and one output port. Typically the front end unit will include a polarization diversity unit for separating the input beam into two sub-beams, and ensuring that the two sub-beams have the same state of polarization. The backend unit for a WB or a DGE can be an array of liquid crystal cells, which independently rotate the state of polarization of the wavelength channels to either partially attenuate or completely block selected channels from passing back through the polarization diversity unit in the front end. Examples of WB and DGE backend units are disclosed in U.S. Pat. No. 7,014,326 issued Mar. 21, 2006 to Danagher et al; U.S. Pat. No. 6,498,872 issued Dec. 24, 2002 to Bouevitch et al; and U.S. Pat. No. 6,810,169 issued Oct. 26, 2004 to Bouevitch, which are incorporated herein by reference.
The arrayed waveguide diffraction grating (AWG) was invented by Dragone by combining a dispersive array of waveguides with input and output “star couplers” on a planar lightwave circuit chip. The AWG can work both as a DWDM demultiplexer and as a DWDM multiplexer, as taught by Dragone in U.S. Pat. No. 5,002,350 (March 1991), which is incorporated herein by reference.
In the interests of reliability and robustness to environmental factors, it is desirable to perform as many of the required functions as possible monolithically on a planar lightwave circuit (PLC). However, there is as yet no practical way of including the MEMS array on the PLC. Accordingly, in one way or another, the wavelength channels from all of the ports must be imaged to a MEMS array of mirrors outside of the PLC.
U.S. Pat. No. 7,027,684 issued Apr. 11, 2006 to Ducellier et al, and United States Patent Publication No. 2004/0252938 published Dec. 16, 2004 to Ducellier et al relate to single and multi-layer planar lightwave circuit (PLC) wavelength selective switches (WSS), respectively, which are illustrated in FIGS. 1 and 2. A single level device 1, illustrated in FIG. 1, includes a PLC 2 with an input AWG in the middle, and a plurality of output AWG's on either side of the input AWG. An input optical signal launched into the input AWG is dispersed into constituent wavelengths, which are directed at different angles through lensing 3 to an array of tiltable mirrors 4. The light is collimated in one direction, e.g. vertically, by a first cylindrical lens 5 adjacent to the PLC 2, while a cylindrical switching lens 6 focuses the output light in the horizontal direction onto the tiltable mirrors 4. Each wavelength channels falls onto a different one of the tiltable mirrors 4, which redirect the individual wavelength channels back through the lensing 3 to whichever output AWG is desired for recombination, and output an output port. For the single level device the tiltable mirrors 4 rotate about a single axis to redirect the wavelength channels within the dispersion plane, i.e. the plane of the PLC 2.
A two level device 11, illustrated in FIG. 2, includes a second PLC 12, similar to the PLC 2, superposed above the PLC 2 with a plurality of input or output AWG's and ports. A second cylindrical lens 15 is superposed above the first cylindrical lens 5 for focusing the beams of light onto the output AWG's provided on the second PLC 12. For the two-level device, tiltable mirrors 14 rotate about two perpendicular axes to redirect the wavelength channels within the dispersion plane (as above) and at an acute angle to the dispersion plane into a plane parallel to the dispersion plane, i.e. the plane of the PLC 12.
In the aforementioned Ducellier devices, the AWG's terminate in straight linear arrays at the edge of the chip, whereby without the curvature at the AWG outputs, the “foci” occur at infinity. Accordingly, an external, bulk-optic lens is required to function as more than simply a field lens, but as a full (spatially) Fourier transforming lens. Consequently, not only is the external lens required to be extremely well aligned, i.e. relatively expensive and extremely sensitive to misalignments, but the optical path is necessarily mostly in air.
An object of the present invention is to overcome the shortcomings of the prior art by providing virtual pupils at the interface between the channel waveguides and the slab waveguide for focusing each wavelength channel at a point outside of the chip.